Optical devices can replace purely electronic components in communications and signal processing applications to enhance both the flexibility and performance of information systems. Replacement of electronic components by optical components motivates the modification of standard optical devices for enhanced performance in new applications.
An example of the modification of a standard optical device for novel applications involves the classical Fabry-Perot cavity. This device usually consists of an optical cavity formed by symmetric partially reflecting mirrors, and it is commonly used to define laser cavities (optical resonators) and for tunable spectrometers. A variation of the Fabry-Perot cavity has been designed by M. Whitehead, et al. (Electronics Letters 25, 566 (1989)); and fabricated by M. Whitehead, et al. (op. cit. 984), and also by R. H. Yan, et al., (Appl. Phys. Lett. 55, 1946 (1989)) to create an electrically controlled single-crystal Asymmetric Fabry-Perot (AFP) reflectance modulator.
The classical Fabry-Perot cavity has no reflection at those wavelengths for which the mirror spacing corresponds to an integral number of half wavelengths. Whitehead and Parry predicted, and their co-workers later demonstrated, that an electric field applied to a cavity which contained a multiple quantum-well (MQW) active region with an appropriate bandgap could produce large changes in the reflectivity of the Fabry-Perot resonator. These changes occur because the field shifts the MQW bandgap, thereby altering the wavelength-dependent absorption and the index of refraction of the active region. This bias-dependent modification of the bandgap occurs through the quantum-confined Stark effect. By electrically altering the properties of the optical cavity, the net reflectivity of the cavity is controled.
The Whitehead and Parry AFP modulator used doped semiconductor mirrors to form a p-i-n structure, so the required reverse bias could be applied to generate the necessary electric field across the quantum-well active region. Also, different periods of semiconductor dielectric mirrors are used to define a Fabry-Perot cavity with a different reflectivity for the top-surface mirror than for the bottom- surface mirror. Through the application of a reverse bias, the bandgap of the quantum wells is reduced, so that photons which previously penetrated the structure are absorbed. Thus, at the appropriate wavelength, the light reflected from the higher reflectivity bottom mirror can be attenuated until its reduced intensity matches that of the light reflected from the lesser reflectivity top mirror. For the proper mirror spacing at the appropriate wavelength, the light reflected from the bottom mirror will be 180 degrees out of phase with that from the top mirror, leading to an electrically controlled cancellation, and thus reduced reflectivity for an appropriate applied bias and cavity design.
The Whitehead and Parry Asymmetric Fabry-Perot device has been realized in AlGaAs/GaAs materials system by R. H. Yan et al., and also in the InGaAs/GaAs system by Pezeshki, et al., (Photonics Technology Letters, 2, 807 (1990)). The AlGaAs/GaAs devices of Whitehead, et al., and of Yan et al., are limited to operating wavelengths in a narrow (20 nm) band near 870 nm for room-temperature operation. Because the GaAs substrate is opaque at the quantum-well wavelengths, the AlGaAs/GaAs AFP devices mentioned above also exclude any applications which require light to be transmitted through the substrate.
The design of Pezeshki, et al. allows the use of wavelengths from 0.87 to 1.0 .mu.m in an Asymmetric Fabry-Perot cavity structure. At these longer wavelengths the GaAs substrate is transparent. However, the manner in which Pezeshki, et al., strain the required, multiple layers of InGaAs to match the in-plane lattice spacing shared by the GaAs substrate and the GaAs and AlGaAs epitaxial layers results in a design that is not thermodynamically stable (e.g., G. A. Vawter and D. R. Myers (J. Appl. Phys., 65, 4769 (1990)). The metastable (i.e., not thermodynamically stable) Pezeshki design can only be realized by highly nonequilibrium growth, and degrades when exposed to energetic ions or to temperatures much above the growth temperature. Additionally, because even the most modern growth technique allows only finite deviations from the limits imposed by equilibrium thermodynamics, too great an indium mole fraction in the quantum wells will lead to dislocation formation during growth, thereby degrading the electrical performance, efficiency, and maximum reflectivity of the device. Pezeshki, et al. do not disclose a device that can operate at 1.06 .mu.m and longer wavelengths.